Steroid hormones cause sex differences in both brain structure and behavior in a variety of species, including humans. Most basic research in the development and display of these differences has been conducted in rodent models such as rats and mice, although there is evidence that the neuroendocrine processes controlling these behaviors in other orders of mammals, particularly primates, differ from those in rodents. Basic research in primates, however, is expensive, time-consuming and often otherwise impractical. The goal of this proposal is to develop an alternative small-animal model to for the study of the sexual differentiation of brain and behavior. The Asian musk shrew Suncus murinus is proposed as the model species because certain features of the regulation of its sexual behavior appear to have more in common with primates than rodents. The proposed studies will examine two behaviors that are different in males and females: copulatory behaviors and spatial learning. Brain structures that are involved in the regulation of these behaviors will also be measured to determine if their development is influenced by sex steroid hormones.